As with alphaiota 244 (q.v.), the headword is feminine accusative singular of this adjective, but in the sense glossed here it comes not from Frogs but from an earlier Aristophanic comedy: Thesmophoriazusae 1054-55; there it agrees with porei/an, i.e. the last journey of the dead. (For a translation see web address 1; 'shortest', i.e. swift, is erroneous, however.) For its many meanings see, besides alphaiota 244, alphaiota 253 and the cross-references there.
The passage here is from a long pastiche of a lament sung by Andromeda in Euripides' tragedy of that name (Andromeda fr. 7 Jouan-van Looy = 122 Nauck, 171a Mette; cf. scholia to Thesm. 1034, 1040). In Aristophanes it is sung by the Relative (of Euripides), acting the part of Andromeda. The gloss occurs in an abridged scholion to Thesm. 1054, but without our headword. The scholia on this section of the comedy are notoriously skimpy. The Suda uses the same gloss for o)rfnai/an, orfnai/hn at kappa 1095, omicron 662.
The gloss is reminiscent of the use of the headword and its related verb in biology for the darkening of a black grape as it ripens (Hesiod, Shield 399; alphaiota 246) and the developing lividity (i.e. black and blue discoloration) of a potentially mortal wound (Sophocles, Philoctetes 1157, with scholia); cf. alphaiota 247 (of night).
For the idea of a shadowy journey to shadowy Hades cf. the scholia to Aeschylus, Seven Against Thebes 856-860a, b; Euripides, Phoenician Women 1484 and scholia. In the very late Orphic Hymn 78.4, Dawn is addressed as "you, who by your risings send to the lower parts of the world night's dark, flesh-discolouring journey (ai)olo/xrwta porei/hn"; some texts read kelaino/xrwta).